Physical activity (PA) has been shown to improve pain and function in persons with knee osteoarthritis (OA), reduce obesity, and prevent the onset and progression of heart disease, diabetes, and chronic pulmonary disease. The US Department of Human and Health Services (DHHS) guidelines recommend that adults engage in >150 minutes of moderate physical activity per week. However, adherence to PA guidelines is poor in the general population, particularly in persons with knee OA. Total knee replacement (TKR) is widely used in patients with symptomatic, advanced knee OA. While the vast majority of persons undergoing TKR experience considerable reduction in pain and improvement in functional capacity, far fewer take this opportunity to become more physically active. Since physical activity has a direct relationship with quality of life and with prevention and amelioratin of many chronic conditions, many TKR recipients do not derive maximum benefits from the procedure. The focus of this proposal is to conduct a proof of concept RCT to establish the efficacy and cost-effectiveness of a behavioral economics-based intervention that would facilitate engagement in physical activity and improve adherence to PA guidelines in the growing population of TKR recipients. We address the innovative hypothesis that the period following TKR presents a window of opportunity to fundamentally change attitudes and beliefs regarding PA, and that tangible economic incentives will effectively induce behavior change and facilitate adherence to PA guidelines. We propose 'Study of Physical Activity Rewards after Knee Surgery (SPARKS)' with the following specific aims: 1. To design and conduct a proof-of- concept RCT to establish the feasibility and efficacy of a personalized intervention built upon the principles of behavioral economics - as compared with standard of care - in improving physical activity among patients who have undergone TKR. 2. To estimate the costs and cost-effectiveness of this behavioral economics intervention to improve physical activity in persons who have undergone TKR. This proof of concept trial will serve as a building block for planning a multicenter RCT and developing an infrastructure to facilitate rewards-based programs to encourage health-promoting behavior. In addition to assessing efficacy we will develop a framework to evaluate the value and budgetary impact of implementing such interventions on a large scale.